OK, first of all you are basically correct that a plasma is composed of
ionized gas. I think the easiest way to start to answer the more subtle
parts of your question is to briefly review the nature of the atom. You
probably
know this stuff, but in clarifying the issues it's good to be sure we are
on common ground. I'll try to keep it brief and straightforward.

The nucleus of an atom is composed of protons, which have a positive
electric charge, and neutrons, which are electrically neutral (thus the
name). The various elements (hydrogen, helium, carbon, etc.) are
differentiated by the number of protons (the amount of positive charge) in
the nucleus. Hydrogen has 1 proton, helium has 2, carbon has 12, uranium
has 92, etc.

Because of the strong attraction between positive and negative charges, a
positively-charged nucleus generally attracts a number of negative
electrons equal to the number of protons, resulting in a zero net charge.
We refer to such a neutral assembly of nucleus and electrons as an atom.

Now, if you put enough energy into the system, it is possible to overcome
the electric attractive force and knock one or more electrons off of the
nucleus. The remaining positive particle, composed of the nucleus plus any
remaining electrons, is referred to as an "ion," and
the act of stripping electrons is called "ionization." Since you haven't
changed the number of protons, you haven't changed the element. Thus, you
can have hydrogen ions, helium ions, carbon ions, etc. Since there are many
electrons on the larger atoms, you can have atoms that are singly-ionized,
doubly-ionized, etc, just indicating how many electrons have been removed.
The energy source for ionization can come from strong electric fields
(sparks/lightning), intense radiation (high-power lasers, particle
accelerators), or particle
collsions (high temperatures).

In the case of a plasma, you have heated a gas so hot that collisions
between the atoms are strong enough to cause ionization. This generally
requires a temperature of thousands of degrees Celsius, otherwise the ions
and electrons quickly recombine into neutral matter. Because the particles
in a plasma have electric charges, the behavior of a plasma is very
different from the behavior of neutral matter - solids, liquids, and gases.
Particles interact with each other over large distances through their
electric fields.

Note that a plasma can still be defined by its elemental components -
hydrogen plasma, helium plasma, carbon plasma, etc. - just like the other
states of matter. The electrons are very light-weight particles, and so
barely even affect the mass of the atom. It isn't until you reach the
temperature of the center of
a star or a nuclear weapon that you start to tear apart or fuse together
the nuclei themselves, forming new elements. Even then, there is nothing
mysterious that happens to the matter. All these processes are pretty well
understood.

OK, now to the "sexy" question. You asked about dark matter/missing mass.
The basic problem is that the matter we can see in galaxies does not appear
to be enough to account for the rotational motion of the galaxies. The
speed
with which they rotate seems to indicate that there is more matter
adding to the gravitational force than we can account for by the
stuff we can see.

The ironic thing about your question is that, if anything, you have the
dark matter problem reversed. The vast majority of matter we can see is in
the plasma state - mostly stars and hot nebulae. Astronomers call this
"luminous" matter, because it is so hot that it glows on its own, allowing
us to see it even over astronomical distances. The tricky matter to detect
in space is what we cold, rock-bound beings consider "normal" matter -
solids, liquids, and gases. Planets, oceans, atmospheres. For the most
part, neutral matter isn't hot enough to glow brightly, so we can only see
it if is is illuminated by a nearby star, or happens to get between us and
a star so we can detect the shadow. Far from being a possible source of the
missing mass, plasmas account for almost all the matter we do know about.

For more on the dark matter hunt, check out the MACHO project (MAssive Compact Halo
Objects). They also have links to other dark matter searches. Some involve
seeking "mundane" dark objects like large planets and brown dwarfs; others
involve searches for new fundamental particles such as WIMPs (Weakly
Interacting Massive Particles) or axions.

For plasma surfing, a great place to start is Plasma on the
Internet. It might be a bit overwhelming at first glance, but there are
links to basic sites as well as plasma-related research programs.